An Overview of Milk Fat Globules

Abstract

Assembly, growth and secretion of MFG takes place in the milk-secreting cells of the mammary gland of mammals. In the original state, tiny intracellular lipid droplets (<0.5 μm) are formed at the endoplasmic reticulum membranes, which is the site of origin of TAGs. These discrete small droplets have a TAG core coated by a single layer of polar lipids and proteins. They migrate from the endoplasmic reticulum to the cytosol, fuse together and form bigger droplets (Heid and Keenan 2005; Deeney et al. 1985). The formation of these cytoplasmic lipid droplets by droplet-droplet fusion is assumed to be governed by calcium and protein complexes originating from the cytosol and fusion-promoting agents, gangliosides (Valivullah et al. 1988). However, the coalescence of cytoplasmic lipid droplets to form larger droplets is not facilitated. It is assumed that the regulation of droplet size might be associated with the difference in composition of surface coat between the micro-lipid and cytoplasmic lipid droplets (Deeney et al. 1985). The lipid droplets are then transported to the apical plasma membrane in which they are discharged from the epithelial cell and secreted. At this point the lipid droplets are progressively coated by the plasma membrane to form the outer bilayer milk fat globule membrane (MFGM), rendering the final trilayer structure of intact MFGM upon secretion (Heid and Keenan 2005) (Fig. 2.1). With its dense protein coat (10–50 nm thick) and complex molecular organization, the MFGM is considered to be a true biological membrane (Keenan and Mather 2006) (Fig. 2.1). The MFGM is enriched in polar lipids and also possesses size-related biochemical and structural differences (Lopez 2011).